Conventionally, as pseudo-grayscale processing for expressing multilevel data in a binary form, an error diffusion method is known (see “An Adaptive Algorithm for Spatial Gray Scale” in Society for Information Display 1975, Symposium Digest of Technical Papers, 1975, p. 36). In this method, letting P be a target pixel, v be the density of the pixel, v0, v1, v2, and v3 be the densities of neighboring pixels P0, P1, P2, and P3 of the pixel P, and T be a threshold for binarization, a binarization error E at the target pixel P is distributed to the neighboring pixels P0, P1, P2, and P3 in accordance with empirically obtained weighting factors W0, W1, W2, and W3 to make the average density macroscopically equal to the density of the original image. In this case, let Do be output binary data, and E be a quantization error.If v≧T, then Do=1 and E=v−VmaxIf v<T, then Do=0 and E=v−Vmin  (1)
(where Vmax: maximum density, and Vmin: minimum density)v0=v0+E×W0  (2)v1−v1+E×W1  (3)v2=v2+E×W2  (4)v3=v3+E×W3  (5)In this case, for example, weighting factors are set as follows: W0= 7/16, W1= 1/16, W2= 5/16, and W3= 3/16.
In performing pseudo-grayscale processing for multiple colors, e.g., four colors, i.e., cyan, magenta, yellow, and black, used in a color ink-jet printer or the like, the above error diffusion method or the like is performed independently for each color. For this reason, although a good visual characteristic is obtained when viewed with respect to a single color component, a good visual characteristic is not always obtained when two or more colors overlap.
In order to solve this problem, pseudo-grayscale processing methods have been disclosed that use the error diffusion method with a combination of two or more colors to obtain a good visual characteristic even where two or more colors overlap (Japanese Patent Laid-Open Nos. 8-279920 and 11-10918). These methods, however, use a fixed binarization threshold for each color, and hence cannot properly cope with a case where different thresholds are set for different colors.
Japanese Patent Laid-Open No. 9-139841 discloses a method of making a similar improvement by independently performing pseudo-grayscale processing for two or more colors and then correcting an output value in accordance with the sum of input values.
The graininess of an intermediate density area of a color image can be effectively reduced by performing image formation so as to prevent dots of cyan components (C) and magenta components (M) from overlapping each other. For this purpose, the following technique is used.
FIG. 6 shows an example of image formation control conforming to a conventional ink-jet scheme.
Assume that image data is expressed by multilevel data with each pixel/each density component (yellow, magenta, cyan, and black: YMCK) consisting of eight bits (gray levels “0” to “255”).
Letting C and M be the density values of the C and M components of an original image, densities Ct and Mt of the C and M components of a target pixel of a multilevel color image to be printed are expressed byCt=C+Cerr Mt=M+Merr where Cerr and Merr are the values error-diffused to the C and M components of the target pixel.
As shown in FIG. 6, in image formation for C and M, image formation control is performed in the following four ways for the obtained values of C and M in accordance with the densities of the C and M components of the target pixel.
1. If the sum of (Ct+Mt) is equal to or less than a threshold (Threshold 1), i.e., the sum belongs to an area 611 in FIG. 6, no dot print operation using C and M inks is performed.
2. If the sum of (Ct+Mt) exceeds the threshold (Threshold 1) and is less than the other threshold (threshold 2), and Ct>Mt, i.e., the sum belongs to an area 612 in FIG. 6, dot print operation is performed by using only C ink.
3. If the sum of (Ct+Mt) exceeds the threshold (Threshold 1) and is less than the other threshold (Threshold 2), and Ct≦Mt, i.e., the sum belongs to an area 613 in FIG. 6, dot print operation is performed by using only M ink.
4. If the sum of (Ct+Mt) is equal to or more than the other threshold (Threshold 2), i.e., the sum belongs to an area 614 in FIG. 6, dot print operation is performed by using both C and M inks.
In this case, Threshold 1<Threshold 2.
Japanese Patent Laid-Open No. 2000-354172 also discloses other error diffusion halftone methods. In one method, in order to make it more difficult for cyan and magenta dots to overlap each other than in the above error diffusion halftone method, where one dot is to be printed, a threshold is modulated on the basis of the sum of cyan and magenta density values so as to make it unlikely for a dot of the other of those two colors to be printed at the same location. In the other error diffusion halftone method, in order to suppress the texture unique to a highlight portion of an image, an error diffusion coefficient that influences dot dispersion properties is changed on the basis of the sum of cyan and magenta density values.
In the above method of individually performing error diffusion for each color, improvements in the avoidance of false edges and in dot dispersion properties have been made. “False edge” is a phenomenon in which dot formation is delayed relative to where it should occur by the accumulation of quantization errors. In the error diffusion halftone method, for example, even on a surface with uniform density, a pixel value may vary across a quantization threshold due to the accumulation of quantization errors. If positive errors are accumulated, a dot is formed. In contrast, if negative errors are accumulated, no dot is formed, resulting in dot omission. In this case, dot formation or omission is delayed until accumulated errors exceed a threshold. This is the so-called false edge phenomenon or discharge phenomenon. Dot dispersion properties literally indicate how dots are dispersed on a printing surface. When dots are unevenly distributed and chained to each other on a highlight portion and a shadow portion, in particular, a texture different from that of the original image may be produced.
Japanese Patent Laid-Open Nos. 8-307669 and 8-307680 disclose error diffusion halftone methods that use a threshold modulation amount set on the basis of the density value of a target color of a target pixel in order to make an improvement in false edge.
Japanese Patent No. 2729278 discloses an error diffusion halftone method that uses an error diffusion coefficient set on the basis of the density value of a target color of a target pixel in order to improve dot dispersion properties.
When, however, a threshold modulation amount set on the basis of the density value of a quantization target color as disclosed in Japanese Patent Laid-Open No. 8-307669 and Japanese Patent Laid-Open No. 8-307680 is applied to the error diffusion method of obtaining a good visual characteristic with respect an overlap of two or more colors as disclosed in Japanese Patent Laid-Open No. 9-139841, an adverse effect such as chaining of dots as shown in FIG. 8 is produced. This is because, in the error diffusion method for an overlap of two or more colors, a threshold for determining on/off control of dots with respect to a quantization target color is not constant and dynamically varies depending on the density values of the other colors unlike the error diffusion method for a single color. That is, for the error diffusion method to be used among a plurality of colors, a threshold modulation amount must be set in consideration of not only the density value of a quantization target color but also the density values of other colors that influence the threshold for the target color.
Consider also dot dispersion properties. When an error diffusion coefficient set on the basis of the density value of a quantization target color is used as disclosed in Japanese Patent No. 2729278, only a faulty dot arrangement like the one shown in FIG. 10 may be obtained. This is also because, in the error diffusion method for overlaps of two or more colors, a threshold for determining on/off control of dots for a target color is not always constant, but dynamically varies depending on the density values of other colors, unlike in the error diffusion method for a single color.
That is, in the error diffusion method to be used among a plurality of colors, in which quantization is performed by comparing the sum of a plurality of color component values with a threshold, in order to realize false edge improvement and dot dispersion property improvement, error diffusion coefficients must be set in consideration of not only the density of a quantization target color but also the density values of other colors which influence a threshold for the color.